The present invention relates to units for supplying sheet material and has been developed with particular attention to its possible use as a unit for supplying sheets of cardboard or card to automatic packaging machines.
In this field of use there is often a need to form pieces from a sheet material of a certain thickness (for example, corrugated cardboard or smooth card) the pieces then being folded so as to be U-, C- or G-shaped or closed up so as to overlap in a generally closed arrangement to form structures for housing and supporting products such as food products. The folded sheet of cardboard or card is usually intended to constitute a support base or, in general, a protective framework for a group of articles packed in a wrapper of thin film material of the type currently known as a "flow-pack", a "form-fill-seal" pack, or simply an "ffs".
Plants which produce wrappers of this type operate at very high speeds (which tend to become ever faster with technological progress) and one aims to make their cycles absolutely continuous by preventing any disturbance leading to the interruption of the operation of the plant, even for a very short time.
One possible disturbance is the periodical running-out of the reel from which the sheet material used in the plant (typically the film which is intended to form the "flow-pack" wrapper, and in the context under consideration, the protective cardboard or card base) is unwound.
Several solutions have already been developed to solve this problem, that is, to prevent the "paper-changing" described above from disturbing the continuous operation of a packaging machine, particularly as regards the sheet material constituted by the thin film which is intended to form the aforementioned flow-pack wrappers. In this connection in particular, several solutions are known in which the end of a reel which is running out can be connected automatically to the beginning of a new reel so that, in practice, the packaging equipment is supplied with wrapping film which is theoretically endless, as a result of the automatic joining of the ends of successive reels of sheet material.
Solutions of this type are generally suitable for use with wrapping film which is very thin and flexible and, in most cases, is constituted by a material which can be welded easily either by heat-sealing or ultrasound welding (these techniques actually being used to seal the ends or flaps of the flow-pack wrappers).
The problem is quite different, however, when the sheet material is constituted by an intrinsically stiffer material such as, for example, corrugated cardboard or card. In particular, with such a material one cannot think simply in terms of automatically connecting the end of a reel which is running out to the beginning of the next reel. This is both because the connection is more difficult to achieve (in fact the materials are usually paper materials and not suitable for heat-sealing or ultrasound welding) and, in particular, because, in most cases, the material in question is quite thick and any connecting region formed by overlapping between two successive reels is in any case of considerable thickness and is readily perceptible as a defect.
In this particular field of use (the changing of the cardboard or card), therefore, semi-automatic solutions are conventionally used.
In practice, at least two supply reels are present in the unit for supplying the cardboard or card at any particular time, one reel acting as the supply reel whilst the other is a spare reel.
An operator notices (possibly because he has been alerted by a warning signal emitted automatically by the unit) that the reel currently in use is about to run out. At this point, the operator intervenes and rapidly unwinds the remaining portion of the sheet of cardboard or card on the reel which is running out so as to make a certain reserve of material available for supplying the machine and then takes the end of the first reel to a work table with which the supply unit is normally provided. The end of the reel which is running out is aligned on the table with the beginning of the new reel which is available close by and the two reels are then joined manually end-to-end by means of adhesive tape. When the packaging machine has used up the reserve formed, it automatically takes in the beginning of the new reel which then takes over the function of the supply reel. The operator can then fit a new reel which will act in turn as a new spare reel in the position previously occupied by the operative reel.
For various reasons, this solution cannot be considered wholly satisfactory.
In the first place, there is a very high probability that the region in which the end-to-end joining of the two ends of two successive reels has been effected will end up in a position in which it is clearly perceptible in the centre of one of the pieces into which the strip of cardboard or card is subsequently cut (usually by a rotary cutter).
In the second place, it is quite easy to unwind the reel which is running out and form a reserve when ribbed or corrugated cardboard is used (naturally with the corrugations or ribs arranged perpendicular to the direction in which the reel is unwound). This is because such a material tends to retain a certain spatial orientation quite well. It is quite difficult, however, to form a reserve with the use of smooth card, which is usually used for small articles.
Moreover, in view of the ever-increasing rates at which packaging machines currently operate, there is a risk that even a fairly large reserve (bearing in mind that the risk of the material in the reserve becoming entangled increases as its length increases) may be used up by the machine within a very short period of time, thus leaving the operator with very few seconds to form the joint.
In any case, the need for the intervention of an operator--in order to ensure the continuity of the operation of the machine rather than simply to replace a used reel with a new reel--is undesirable since, if the person responsible is distracted or temporarily unavailable for some reason at a certain moment, there is a risk of a stoppage of the machine.
The object of the present invention is therefore to provide a unit for supplying sheet material such as, for example, cardboard or card, which completely overcomes all the problems mentioned above.
According to the present invention, this object is achieved by virtue of a unit for supplying sheet material having the specific characteristics recited in the following claims.
The present invention is based, essentially, on the idea of incorporating two supply stations in cascade in the unit for supplying the sheet material, the two stations being able to supply the desired pieces of cardboard or card to a common output line (which is usually situated below them) and each station automatically taking over the supply operation from the other when the reel of sheet material in one of the stations runs out.